CREATE FUNCTION ret_date (crdate date) RETURNS text AS $$ 
BEGIN 
   IF EXTRACT (MONTH FROM crdate)= 1 
   THEN RETURN 'January'; 
   ELSIF EXTRACT (MONTH FROM crdate)= 2 
   THEN RETURN 'February'; 
   ELSIF EXTRACT (MONTH FROM crdate)= 3 
   THEN RETURN 'March'; 
   ELSIF EXTRACT (MONTH FROM crdate)= 4 
   THEN RETURN 'April'; 
   ELSIF EXTRACT (MONTH FROM crdate)= 5 
   THEN RETURN 'May'; 
   ELSIF EXTRACT (MONTH FROM crdate)= 6 
   THEN RETURN 'June'; 
   ELSIF EXTRACT (MONTH FROM crdate)= 7 
   THEN RETURN 'July'; 
   ELSIF EXTRACT (MONTH FROM crdate)= 8
   THEN RETURN 'August'; 
   ELSIF EXTRACT (MONTH FROM crdate)= 9 
   THEN RETURN 'September'; 
   ELSIF EXTRACT (MONTH FROM crdate)= 10 
   THEN RETURN 'October'; 
   ELSIF EXTRACT (MONTH FROM crdate)= 11
   THEN RETURN 'November'; 
   ELSIF EXTRACT (MONTH FROM crdate)= 12
   THEN RETURN 'December'; 
   END IF; 
END; 
$$ 
LANGUAGE plpgsql;

SELECT ret_date(CURRENT_DATE);
select ret_date('2017-04-05');